The vein of invention McCaffrey worked so effectively in her Pern series seems to have been exhausted. Collaborating again with Ball (after Partnership), McCaffrey opens promisingly in the far future with charming doomed unicorn beings who seal their infant into a survival pod, hoping someone will save her after they choose to die in space rather than in a grisly Khlevii torture cell. After three grungy Terran bachelor asteroid miners find the silver-curled, long-faced baby and name her ""Acorna"" for the strange protuberance growing from her forehead, the story gallops into a gulch of sentimentality. Acorna's horn can detect poisons and nuzzle sick and wounded humans back to health, so she becomes the savior of Kezdet, a Dickensian planet full of abused children slaving in mines, match factories and brothels. The authors stall in getting their major theme of exploited children under way, and they unconvincingly muddle it with precious goings-on among Acorna's three adopted miner dads, sentimentalized little victims, shady planetside entrepreneurs and a stock villain. Cut the ""a""s from the title and what's left sums up this novel perfectly. (July)